INFLUENCE OF ELECTRICAL CURRENTS. 493 



action of the gas, possibly through drying. The results may be com- 

 pared with those obtained by Waller in isolated nerve, in which the 

 electrical response is taken as the index of nerve excitation ; these are 

 referred to in detail in the section dealing with the electromotive 

 phenomena. 



Influence of electrical currents upon nerve excitability and con- 

 ductivity — Polarisation changes. — The passage of electrical currents 

 through a nerve causes profound changes in both its excitability and 

 its conductivity. Since the nerve is a moist conductor, such passage 

 produces electrolytic changes, which are most pronounced at the poles, 

 i.e.. the point of entry (anode) and point of exit (cathode). They spread 

 in diminishing amount from these situations along the nerve fibres. 

 These polarisation changes are presumably based upon the formation 

 of electrolytic products or ions ; they manifest themselves during closure 

 by the establishment of an increased electrical resistance and extrapolar 

 currents, on opening by the establishment of counter electromotive 

 changes or polarisation currents, which in the intrapolar region are of 

 opposite sign to the original or polarising current. The formation 

 of ions, and thus the polarisation, may be distinguished broadly 

 according to its locality as external and internal. When a current 

 is led through a nerve by means of metallic electrodes, an exten- 

 sive electrolytic change occurs at the actual points of metallic contact. 

 The changes may be so pronounced as to produce on the opening 

 of the polarising current a counter polarisation current of sufficient 

 intensity to excite the nerve. Hence, if by means of metallic elec- 

 trodes a galvanic current is led through the sciatic nerve of the frog, 

 then, on the battery being disconnected, the closure and opening of the 

 connections attached to the electrodes is accompanied by an excitation 

 indicated by a muscular response. 



Such external polarisation must mask the effect of any similar 

 change in the nerve fibres themselves, and must be eliminated or 

 reduced to very small proportions in all experiments in which electrical 

 currents are used for nerve stimulation. It is reduced by employing 

 platinum electrodes and currents of very short duration, and by arranging 

 that, when a rapid series of these are used, they shall be alternate in 

 direction, and as far as possible of similar duration and intensity. The 

 make and break induced currents obtained by Helmholtz's modification 

 (the side wire of the induction coil), are employed in order to attain 

 this end. It is still further reduced by employing non-polarisable 

 electrodes. 



The most common form of such electrodes is that in which the metallic 

 wire terminates in a zinc rod dipping into saturated sulphate of zinc, and this 

 is connected to the nerve by means of an intervening moist conductor of 

 physiological salt solution. Kaolin forms a convenient non-polarisable medium 

 for the retention of the liquids, as it can be made into a paste and moulded 

 to any desired shape. The actual contacts may be such paste, fine brushes, 

 threads, or filter paper, steeped with the salt solution, or plastered by paste 

 similarly steeped. Silver wires coated with a fine layer of fused silver chloride, 

 and then covered with filter paper steeped in physiological salt solution, have 

 been employed by D'Arsonval for similar purposes. Although advantageous 

 in many respects, these electrodes offer a very large resistance, and are difficult 

 to make as free from polarisation as the other form. 



When a current is led through a nerve by non-polarisable electrodes, 



